The present invention relates to glue gap control in glue machines. In particular, the present invention relates to automated servo control of the gap between a glue roll and a metering roll in a glue machine to achieve a desired adhesive film thickness on the glue roll.
Glue machines find widespread use in the construction of packing materials, including, for example, corrugated paperboard. In one type of corrugated paperboard construction process, a first single faced sheet of corrugated paperboard moves through a glue machine that applies glue to flute tips of the single faced sheet. Subsequently, a second opposing sheet of corrugated paperboard is mated by the applied glue to the first sheet to form a doublebacked sheet of corrugated paperboard. The doublebacked sheet may then be formed into boxes, tubes, and other packing containers.
The glue machine typically incorporates a glue roll with a surface that rotates through a glue reservoir to pickup or accumulate glue. A metering mechanism adjacent to the glue roll attempts to restrict the adhesive film thickness present on the glue roll (and thus the amount of glue applied to the first sheet of corrugated paperboard). The metering mechanism may vary between glue machines, and may be a glue roll, a scraper blade, or the like.
Generally, the most significant factor in controlling the adhesive film thickness (and thus the amount of glue applied to the first sheet of corrugated paperboard) is the glue gap size. The glue gap size is the distance between the metering mechanism and the glue roll. The glue gap size has a direct influence on the adhesive film thickness on the glue roll, and as a result, is an extremely important parameter in ensuring that the glue roll applies the appropriate amount of glue to the first sheet.
In the past, however, glue machines did not exercise precise control over the glue gap. As one example, some prior glue machines incorporated a hand wheel that an operator used to vary the size of the glue gap. Not only was operator adjustment of the hand wheel relatively inexact, but it also required the operator to manually inspect the resulting amount of glue applied to the first sheet of corrugated paperboard. Thus, manual adjustment of the hand wheel generally was not undertaken more than once or twice per day, and often resulted in application of an inappropriate amount of glue to the first sheet.
In particular, the glue roll generally applied too much glue for most applications. With too much glue between the first and second sheets of corrugated paperboard, the paperboard took far longer to dry than it would have given an appropriate amount of applied glue. Furthermore, a glue roll that overapplied glue was wasteful and also increased the cost of producing the doublebacked sheet of corrugated paperboard.
Another attempt at glue gap control included providing a reversible AC motor, encoder, and programmable logic controller (PLC). In turning under control of the PLC, the AC motor, coupled to the glue roll through a mechanical linkage, adjusted the size of the glue gap. Such approaches used low resolution encoders and thus were relatively inexact as well. Furthermore, the PLC and encoder added significantly to the cost of implementing this type of glue gap control. Additionally, sophisticated control and feedback of the motor acceleration, velocity, position, torque, and the like, at each point in time (useful, for example, for compensating for mechanical slop and sensing jams) were not available.
A need has long existed in the industry for a method and apparatus for glue gap control that addresses the problems noted above and others previously experienced.
A preferred embodiment of the present invention provides a servo control system for adjusting adhesive film thickness on a glue machine glue roll. The control system includes a servo motor for driving a glue gap adjusting mechanical linkage, a position encoder coupled to the servo motor, and a servo position controller coupled to the position encoder. In addition, an operator control interface coupled to the servo position controller directs the operation of the servo motor.
In one embodiment, the operator control interface includes a processor and a memory storing instructions for execution by the processor. The instructions are responsive to position encoder feedback for turning the servo motor to achieve a desired glue gap size. Optionally, the instructions may turn the servo motor in accordance with a selected velocity profile for the servo motor. The velocity profile typically specifies servo motor velocity between a start position (e.g., a current glue gap size) and an end position (e.g. the desired glue gap size).
The control system may determine the desired glue gap size as a function of many glue machine parameters. The function, however, is preferably an approximately linear function of the glue machine line speed. Alternatively, the control system may consult a preprogrammed lookup table of glue machine line speeds, desired adhesive film thickness, glue gap sizes, and other parameters to determine the desired glue gap size.
Using feedback from the position encoder, the control system may turn the motor to achieve the desired glue gap size, then back away from the desired glue gap size, then turn back into the desired glue gap size. In this manner, the control system may compensate for gear backlash or mechanical slop. Furthermore, the memory may store instructions that monitor a servo motor torque feedback signal for jam sensing.